F324 Rings, Polymers and Analysis
Practical Assessment 3
(b)
The preparation of an organic liquid compound (e.g. the preparation of an ester). It
is also expected that candidates will be aware of the procedures involved in the
purification and re-distillation at an appropriate temperature of the product obtained.
The Preparation of Ethyl Ethanoate
Reaction
1) Mix 50cm3 of ethanol and 50cm3 of glacial ethanoic acid thoroughly in a 250cm3
round-bottomed flask.
2) Add slowly with cooling and shaking 10cm3 of concentrated sulphuric acid.
3) Ensure that the mixture is homogeneous, then fit the flask with a reflux watercondenser and boil the mixture gently for 10 minutes.
Separation of the product
4) Rearrange the position of the condenser for distillation, and distil off about two-thirds
of the mixture.
5) Transfer the distillate to a separating funnel and add about 25 cm3 of 30% sodium
carbonate solution. Stopper the funnel, invert it, and shake, opening the tap from time
to time.
6) Allow the two layers to separate and carefully run off and reject the lower layer,
ensuring that the sodium carbonate is removed as completely as possible.
Purification of the Product (you do not need to carry out this part)
7) Add some saturated calcium chloride solution to the ethyl ethanoate, shake well, and
remove the lower aqueous layer.
8) Pour the ethyl ethanoate into a beaker, add a few lumps of solid anhydrous calcium
chloride and shake well. Decant the ethyl ethanoate into a flask.
9) Then decant the liquid into a clean, dry 100 cm3 round-bottom flask, add some antibumping granules, and arrange for distillation including a 0-100oC thermometer in the
apparatus. The distilling flask should be placed in a cold water bath, which is
gradually heated. The ether that is always formed in this reaction will distil off at 3540oC, and may be discarded. Continue to heat, and collect the fraction that boils
between 74o and 79oC.
Questions:
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Why must the concentrated sulphuric acid be added slowly and with cooling?
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Why is concentrated sulphuric acid used?
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Why must the mixture be homogeneous?
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What is a reflux water-condenser?
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Why is the reaction comparatively slow?
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What is the function of the sodium carbonate solution?
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Why do you need to open the tap from time to time?
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What is the function of the calcium chloride solution?
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What is the function of the solid calcium chloride?
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Why should the distillation flask be clean and dry?
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What is the nature and purpose of anti-bumping granules?
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Why is an ether formed in this reaction?
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Explain the meaning of the term glacial as applied to ethanoic acid.
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Glacial ethanoic acid is pure ethanoic acid CH3COOH. It is called glacial because on
cooling it forms glassy crystals.
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Why must the concentrated sulphuric acid be added slowly and with cooling?
The concentrated sulphuric acid on dilution gives out a lot of heat; the slow addition
with cooling is necessary to avoid splashing if the mixture gets hot.
Why is concentrated sulphuric acid used?
Concentrated sulphuric acid is a catalyst for the esterification reaction
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CH3CH2OH + CH3COOH
CH3COOCH2CH3 + H2O
Why must the mixture be homogeneous?
Concentrated sulphuric acid is much denser than any of the other reagents. If not well
mixed initially, the solution is liable to get too hot and boil uncontrollably when
mixing occurs later in the reaction.
What is a reflux water-condenser?
A reflux water condenser is a Liebig condenser arranged vertically above the reaction
flask; vapors are condensed and returned to the flask, the contents of which can
therefore be boiled for long periods without any loss of material.
Why is the reaction comparatively slow?
Most organic reactions are slow since they involve the breaking of strong covalent
bonds. The proportion of molecular collisions that have the necessary activation
energy is usually fairly low.
What is the function of the sodium carbonate solution?
The distillate contains traces of ethanoic acid and perhaps some sulphuric acid.
Sodium carbonate solution removes this.
Why do you need to open the tap from time to time?
The neutralisation with sodium carbonate produces carbon dioxide gas; opening the
tap releases this and avoids a build-up of pressure that might blow the stopper out of
the funnel.
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What is the function of the calcium chloride at this stage of the preparation?
The crude ethyl ethanoate contains traces of ethanol; the calcium chloride solution
removes this since it complexes with the ethanol
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What is the function of the calcium chloride at this stage of the preparation?
The solid calcium chloride is a dehydrating agent; it removes any remaining traces of
water from the product.
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Why should the distillation flask be clean and dry?
You have just dried the mixture; you don't want it wet again.
What is the nature and purpose of anti-bumping granules?
Anti-bumping granules are small pieces of silica; broken unglazed pottery works as
well. This provides a nucleus on which gas bubbles grow, therefore avoiding the
sudden production of large gas bubbles which can lead to 'bumping'. This, properly
called succussion, causes liquid to splash over into the condenser and therefore produce an
impure product. Severe bumping can even lead to loss of material through vents or can blow
a distillation apparatus apart.
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Why is an ether formed in this reaction?
An ether is formed because sulphuric acid can cause the elimination of a molecule of
water between two alcohol molecules. The water causes the ionisation of the
sulphuric acid:
CH3CH2OH + HOCH2CH3 + H2SO4 
CH3CH2OCH2CH3 + H3O+ + HSO4-